The sex lives of animals: A rough guide

Transsexualism

Changing sex is a tricky business for humans, involving hormone injections and surgery, but some animals make it look easy. Transsexualism is particularly common among simple creatures like worms and slugs, but also among fish.

Some species engage in "sequential hermaphroditism", in which they start out one sex, but switch later in life. One example is a marine worm called Ophryotrocha puerilis puerilis, which starts out male but may become female. Large males are particularly likely to do so, as being large is an advantage for the females, but not the males.

Sex changes don't have to be one-way. When two female gobi fish meet, one of them will often become male so that they can mate. Switching back is less common, perhaps because there are higher costs involved, but it does happen.

Many animals have their cake and eat it by practising "simultaneous hermaphroditism" - they have both male and female sexual organs. Snails and earthworms both do this. Sea slugs have it down to a fine art, engaging in "sperm trading" to ensure that both partners get their fare share.

Self-fertilisation is less common than mutual mating - neither snails nor earthworms do it. Banana slugs, which live in North America and reach lengths of 25 centimetres, have no such inhibitions and regularly fertilise themselves. Only one fish species, the mangrove killifish, is known to self-fertilise.

Part 2

We now move on to sexual acts that many of us would consider "alternative", even perverse.

It is difficult to equate the behaviours observed in animals with those humans engage in. Humans who indulge in these activities often do it partly for the thrill of breaking taboos, or because they delight in doing things they believe to be morally wrong.

By contrast, animals doing them may simply be making mistakes, or attempting to survive in difficult circumstances. It is unlikely that they have the same complex emotional response to, say, bondage, as a human has.

Violence and forced sex

It should come as no surprise that sex in the animal kingdom is often rough and dangerous. Female praying mantises are the poster child of dangerous sex, thanks to their predilection for biting off their mates' heads immediately after copulation (watch a video) - but many seemingly mild-mannered creatures can also be pretty violent in their sex lives.

For instance, short-tailed shrews are not normally thought of as a brutal species. However, if a male persists in attempting to mate with an unreceptive female, a fight may break out - and such fights can end with one of the shrews being killed and eaten by the other (pdf format, see page 11).

Weapons are not out of bounds in the animal world, either. Male guppies mate using a hooked appendage called a gonopodium. This maims the female, causing inflammation which locks the male's sperm inside. Male guppies will also mate with females of another species called Skiffia, which have no defences against the damage the gonopodium can cause.

Some sort of prize ought to be given, however, to the flatworm Pseudoceros bifurcus. These hermaphroditic worms simply stab one another with their penises, leaving both parties severely punctured.

Bondage

Among humans, tying up a sexual partner is a form of thrill-seeking, but for male spiders it can be a life-saver.

Mating among spiders is notoriously tricky, as the male is often smaller than the female and may well end up getting eaten. Males have developed all manner of techniques to get around this problem, including playing dead.

But one of the sneakiest tricks is deployed by, among others, European crab spiders. The males tie down their prospective mates with silk threads to ensure they don't become a post-copulation snack.

An unfortunate male duck had crashed into the window of the Rotterdam museum - at which Moeliker worked - and lay dead on the ground. At this point another male approached and mated with the dead duck for over an hour.

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